Electrically heated burner

ABSTRACT

A burner includes an electrically powered heater configured to output heat energy to a burner portion configured to contact a fuel stream or a combustion reaction supported by the fuel stream.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. Continuation-in-Part application whichclaims priority benefit under 35 U.S.C. §120 (pre-AIA) of co-pendingInternational Patent Application No. PCT/US2015/016456, entitled“ELECTRICALLY HEATED BURNER,” filed Feb. 18, 2015 (docket number2651-267-04). Co-pending International Patent Application No.PCT/US2015/016456 claims priority benefit from U.S. Provisional PatentApplication No. 62/104,028, entitled “ELECTRICALLY HEAT ANY FLAMEHOLDER,” filed Jan. 15, 2015 (docket No. 2651-267-02). Co-pendingInternational Patent Application No. PCT/US2015/016456 claims priorityto International Application No. PCT/US2014/016632, entitled “FUELCOMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER,” filed Feb. 14,2014 (docket number 2651-188-04). The present application is also aContinuation-in-Part of co-pending U.S. patent application Ser. No.14/763,271, entitled “PERFORATED FLAME HOLDER AND BURNER INCLUDING APERFORATED FLAME HOLDER,” filed Jul. 24, 2015 (docket number2651-172-03). Co-pending U.S. patent application Ser. No. 14/763,271claims priority benefit to International Patent Application No.PCT/US2014/016628, entitled “PERFORATED FLAME HOLDER AND BURNERINCLUDING A PERFORATED FLAME HOLDER,” filed Feb. 14, 2014 (docket number2651-172-04). International Patent Application No. PCT/US2014/016628claims the benefit of U.S. Provisional Patent Application No.61/765,022, entitled “PERFORATED FLAME HOLDER AND BURNER INCLUDING APERFORATED FLAME HOLDER,” filed Feb. 14, 2013 (docket number2651-172-02). The present application is also a Continuation-in-Part ofco-pending U.S. patent application Ser. No. 15/215,401, entitled “LOWNO_(X) FIRE TUBE BOILER,” filed Jul. 20, 2016 (docket number2651-205-03). Co-pending U.S. patent application Ser. No. 15/215,401claims priority benefit to International Patent Application No.PCT/US2015/012843, entitled “LOW NO_(X) FIRE TUBE BOILER,” filed Jan.26, 2015 (docket number 2651-205-04). International Patent ApplicationNo. PCT/US2015/012843 claims the benefit of U.S. Provisional PatentApplication No. 61/931,407, entitled “LOW NO_(X) FIRE TUBE BOILER,”filed Jan. 24, 2014 (docket number 2651-205-02). Each of theinternational patent applications, U.S. patent applications, and U.S.provisional patent applications listed in this paragraph are, to theextent not inconsistent with the disclosure herein, incorporated byreference.

SUMMARY

According to an embodiment, an electrically heated burner includes aburner portion configured for contact with at least one of a fuel streamor a flame and an electrically powered heater operatively coupled to theburner portion. A power supply can be operatively coupled to theelectrically powered heater. A controller can be configured to control aflow of current from the power supply to the electrically poweredheater. The electrically powered heater can be configured to output heatenergy to the burner portion to cause a temperature of the burnerportion to be controlled proportional to the controlled current flow.

According to an embodiment, a burner includes a fuel and oxidant sourceconfigured to cooperate to produce a fuel and oxidant stream thatincludes a range of fuel concentrations. A body is disposed to receivethe fuel and oxidant stream, configured to maintain combustion withinperforations of the body, and to exchange heat energy with thecombustion reaction therein. An electrically powered heater isoperatively coupled to the body. A current source powers theelectrically powered heater. A thermostatic control circuit selectivelycouples the current source to the electrically powered heater to holdthe body at a selected temperature.

According to another embodiment, a conventional flame holder body isdisposed adjacent to a fuel and oxidant stream and is configured toinitiate and maintain vortices in the fuel and oxidant stream. Anelectrically powered heater is configured to raise the conventionalflame holder body to an elevated temperature. The conventional flameholder body is configured to exchange heat energy with an adjacentcombustion reaction supported by the fuel and oxidant stream.

According to another embodiment, a method of combustion includesoutputting fuel into a combustion volume, admitting air into thecombustion volume, and allowing the fuel and air to mix in thecombustion volume to form a fuel and air jet having a range of mixtures.An igniter ignites the fuel and air jet to initiate combustion. A flameholder is supported adjacent to the fuel and air jet or to receive thefuel and air jet so has to hold the combustion reaction in apredetermined position. The flame holder is electrically heated.Ignition of the fuel and air mixture is maintained by heat exchange withthe electrically heated flame holder.

According to another embodiment, a control system controls dynamics of aflame. The control system includes a source of electric current and anelectrically-heated flame holder positioned to be in contact with theflame and operatively coupled to the source of electric current. Thesource of electric current induces electric current flow in anelectrical heater operatively coupled to the electrically-heated flameholder. A temperature-responsive current controller coupled to thecurrent source and the electrically-heated flame holder adjusts theelectric current flow through the electrically-heated flame holder todrive the electrically-heated flame holder to a predeterminedtemperature.

According to an embodiment, a method for operating an electricallyheated burner includes supporting combustion adjacent to at least aportion of an electrically heated industrial or commercial burner,outputting electrical current from a power supply to an electricallypowered heater, causing the electrically powered heater to dissipateheat energy responsive to outputting the electrical current, and raisinga temperature of the portion of the electrically heated burner with theheat energy.

According to another embodiment, a method for operating an electricallyheated burner includes receiving start-up command data into a controllervia a data interface, enabling a relay to output a drive voltage on atleast one node of a power supply, receiving the drive voltage at anelectrically powered heater, outputting heat energy from theelectrically powered heater to a component of an industrial orcommercial burner to cause a sensible temperature rise in the burnercomponent and contacting a fuel stream or a flame supported by the fuelstream with the burner component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an electrically heated burner, according to anembodiment.

FIG. 2 is a simplified perspective view of a burner system including aperforated flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flameholder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner systemincluding the perforated flame holder of FIGS. 1, 2 and 3, according toan embodiment.

FIG. 5 is a partially schematic, partially cross-sectional diagram of asystem for controlling electric heating of a flame holder, according toan embodiment.

FIG. 6 is an elevational, partially cross-sectional diagram of aflame-holding burner tile with controllable fuel and air feeds,according to an embodiment.

FIG. 7 is a cross-sectional diagram of a flame holder with an electricheater element inserted therethrough, according to an embodiment.

FIG. 8 is a diagrammatic perspective view of a combustion system in afurnace with feedback control of fuel flow and electric heating,according to an embodiment.

FIG. 9 is a flow chart showing a method for operating an electricallyheated burner, according to an embodiment.

FIG. 10 is a flow chart showing a method for operating an electricallyheated burner, according to another embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here.

FIG. 1 is a diagram of an electrically heated burner 100, according toan embodiment. Various embodiments of the electrically heated burner 100are contemplated. For ease of understanding, FIG. 1 depicts a sectionalview, augmented by a block diagram, of an electrically heated burner 100including an otherwise conventional burner tile 104 with an addedelectrically powered heater 106, according to an embodiment.

The burner tile 104 can be a type that can be used to heat a furnace,duct, oven, or boiler volume 108 arranged to receive combustion heatfrom the electrically heated burner. The burner tile 104 can be of atype that is typically formed from a refractory material such as afiber-reinforced refractory or cementatious material formed as a bluffbody flame holder configured to hold a combustion reaction 110, shown asa flame, in the furnace, duct, oven, or boiler volume 108. According toembodiments, primary fuel nozzles 112 and secondary fuel nozzles 114 arearranged to respectively impinge at least partially on interior andexterior surfaces 116, 118 of an axially symmetric burner tile 104. Theburner may receive combustion air through a central natural or forceddraft air source 120 and/or from other sources of combustion air. As isknown to the art, the conventional aspects of the burner 100 supportstaged combustion as an attempt to control output of undesirablecombustion products such as oxides of nitrogen (NOx) and carbon monoxide(CO).

One function of the burner tile 104 can be to provide flame holding forthe combustion reaction 110. The inventors note that, especially in lowNOx burners, flame holding can be somewhat tenuous and tricky tomaintain. Other shortcomings are apparent to those skilled in the art.

To address the shortcomings and design trade-offs of burners 100 used incommercial and industrial applications, the inventors believe thatelectrically heating a portion of the burner 100 such as the burner tile104 offers performance, capital cost, and/or operating cost advantages.The inventors contemplate use of the electrically powered heater 106during system start-up, for example, to reduce time during which theburner 100 must be held at a low heat output. The inventors contemplateuse of the electrically powered heater 106 during steady-stateoperation, for example, to improve stability of flame 110 holding by theburner tile 104. For example, the inventors have noted a tendency forflame lift-off and other asymmetries in existing burners under someoperating conditions. For another example, the inventors contemplate useof the electrically powered heater 104 to improve system turn-downrange. Conventional burners can exhibit a limitation with respect to theminimum amount of heat output compared to maximum heat output,especially in “low NOx” and “ultra low NOx” burners (both of whichtypically output far more NOx and/or more CO than a burner 200 withperforated flame holder 102 described in FIGS. 2-3 below). In anembodiment, electrically heating the burner tile 104 can help tomaintain stable combustion even when fuel flow rate is reduced to alevel insufficient to maintain a preferred operating temperature of theburner tile 104. In some embodiments, the inventors contemplate use ofthe electrically powered heater 106 to maintain the burner 100 in a“warm stand-by” mode wherein fuel flow is stopped during periods of lowdemand for heat release. This approach can afford the ability to startup the burner 100 more quickly to respond to an increase in powerdemand, process throughput, etc.

During start up, an igniter 122, such as a hot surface igniter,electrical discharge igniter, pilot flame, etc. can be used to ignitethe primary fuel stream from the primary fuel nozzles 114. In typicalprior art systems, only primary fuel is output for a time sufficient toheat the surface, and particularly a flame-holding surface 124sufficiently to hold a larger flame supported by secondary fuel from thesecondary fuel nozzles 114. During this time, the burner 100 cannotrespond as quickly as may be desired to surge demand and/or theconcentration of undesirable combustion products such as NOx and CO canbe elevated.

According to an embodiment, during start up, a controller 126 controlsan electrical current source 128 to energize a primary fuel igniter 122and to energize the electrically powered heater 106. The additional heatoutput by the electrically powered heater 106 causes extra heat energyto be dissipated to the burner tile, and allows for faster start up. Inan embodiment, the primary fuel igniter 122 and the electrically poweredheater 106 can be energized at different and/or overlapping periods oftime. In an embodiment, the controller 126 causes the electrical currentsource 128 to energize the electrically powered heater 106 upon receiptof a start up command via a data interface 130. A timer and/or optionalone or more sensor(s) 132 operatively coupled to the controller candetermine when the burner tile 104, and especially a flame holdingsurface 124 of the burner tile 104 is heated sufficiently for optimumignition of the primary flame. Upon reaching a start up temperature, thecontroller 126 can cause the current source 128 to energize the igniter122 and can cause a primary fuel valve 134 to open to supply fuel flowto the primary fuel nozzles 114 and ignite a primary flame. At a latertime or simultaneously, the controller 126 can cause a secondary fuelvalve 136 to open to supply fuel flow to the secondary fuel nozzles 114.Owing to the electrical heating of the burner tile 104 by theelectrically powered heater 106, the time delay between receiving astart up command via the data interface 130 and igniting primary andsecondary combustion 110 can be decreased, compared to a nonelectrically heated burner.

According to an embodiment, during steady-state operation, thecontroller can receive a temperature signal from temperature sensor(s)132 (which may optionally be embedded in the burner tile 104 and/orembodied as an electrical resistance measurement across the power supplynodes 138, 140) and cause output of current from the current source 128to maintain a desired temperature. In this way, the controller 126 andsensor(s) 132 can operate as a thermostat that maintains a temperatureof at least the flame holding surface 124 of the burner tile 104.

According to an embodiment, a thermostat is configured to sense thetemperature of the burner portion(s) and control current delivered fromthe power supply to the electrically powered heater to maintain a rangeof temperatures near a temperature set point. In one embodiment thethermostat has a precision of plus or minus 5 degrees F. In anotherembodiment, the thermostat has a precision of plus or minus 2 degrees F.In another embodiment, the thermostat has a precision that is poorerthan plus or minus 2 degrees F.

According to an embodiment, the temperature sensor is operativelycoupled to the burner portion and the controller, and configured tomeasure a temperature of the burner portion, and to output a temperaturesignal or data corresponding to the measured temperature to thecontroller.

According to another embodiment, the controller includes a timercircuit, a non-transitory computer readable medium carrying datacorresponding to a duty cycle set point, and a signal generatoroperatively coupled to the timer circuit and configured to generate anelectrical heater control signal having a duty cycle corresponding tothe duty cycle set point. The controller can thus be configured tocontrol the current flow from the power supply to the electricallypowered heater according to the electrical heater control signal.

While the description corresponding to FIG. 1 was directed to anembodiment wherein the electrically powered heater 106 was operativelycoupled to an otherwise conventional burner tile, the inventorscontemplate additionally or alternatively providing an electricallypowered heater to a perforated flame holder 102, described in moredetail below in conjunction with FIGS. 2-4.

FIG. 2 is a simplified diagram of a burner system 200 including aperforated flame holder 102 configured to hold a combustion reaction,according to an embodiment. As used herein, the terms perforated flameholder, perforated reaction holder, porous flame holder, porous reactionholder, duplex, and duplex tile shall be considered synonymous unlessfurther definition is provided.

Experiments performed by the inventors have shown that perforated flameholders 102 described herein can support very clean combustion.Specifically, in experimental use of systems 200 ranging from pilotscale to full scale, output of oxides of nitrogen (NOx) was measured torange from low single digit parts per million (ppm) down to undetectable(less than 1 ppm) concentration of NOx at the stack. These remarkableresults were measured at 3% (dry) oxygen (O₂) concentration withundetectable carbon monoxide (CO) at stack temperatures typical ofindustrial furnace applications (1400-1600° F.). Moreover, these resultsdid not require any extraordinary measures such as selective catalyticreduction (SCR), selective non-catalytic reduction (SNCR), water/steaminjection, external flue gas recirculation (FGR), or other heroicextremes that may be required for conventional burners to even approachsuch clean combustion.

According to embodiments, the burner system 200 includes a fuel andoxidant source 202 disposed to output fuel and oxidant into a combustionvolume 204 to form a fuel and oxidant mixture 206. As used herein, theterms fuel and oxidant mixture and fuel stream may be usedinterchangeably and considered synonymous depending on the context,unless further definition is provided. As used herein, the termscombustion volume, combustion chamber, furnace volume, and the likeshall be considered synonymous unless further definition is provided.The perforated flame holder 102 is disposed in the combustion volume 204and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforatedflame holder 102 of FIGS. 1 and 2, according to an embodiment. Referringto FIGS. 2 and 3, the perforated flame holder 102 includes a perforatedflame holder body 208 defining a plurality of perforations 210 alignedto receive the fuel and oxidant mixture 206 from the fuel and oxidantsource 202. As used herein, the terms perforation, pore, aperture,elongated aperture, and the like, in the context of the perforated flameholder 102, shall be considered synonymous unless further definition isprovided. The perforations 210 are configured to collectively hold acombustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporizedhydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered orpulverized solid. The fuel can be a single species or can include amixture of gas(es), vapor(s), atomized liquid(s), and/or pulverizedsolid(s). For example, in a process heater application the fuel caninclude fuel gas or byproducts from the process that include carbonmonoxide (CO), hydrogen (H₂), and methane (CH₄). In another applicationthe fuel can include natural gas (mostly CH₄) or propane (C₃H₈). Inanother application, the fuel can include #2 fuel oil or #6 fuel oil.Dual fuel applications and flexible fuel applications are similarlycontemplated by the inventors. The oxidant can include oxygen carried byair, flue gas, and/or can include another oxidant, either pure orcarried by a carrier gas. The terms oxidant and oxidizer shall beconsidered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can bebounded by an input face 212 disposed to receive the fuel and oxidantmixture 206, an output face 214 facing away from the fuel and oxidantsource 202, and a peripheral surface 216 defining a lateral extent ofthe perforated flame holder 102. The plurality of perforations 210 whichare defined by the perforated flame holder body 208 extend from theinput face 212 to the output face 214. The plurality of perforations 210can receive the fuel and oxidant mixture 206 at the input face 212. Thefuel and oxidant mixture 206 can then combust in or near the pluralityof perforations 210 and combustion products can exit the plurality ofperforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 isconfigured to hold a majority of the combustion reaction 302 within theperforations 210. For example, on a steady-state basis, more than halfthe molecules of fuel output into the combustion volume 204 by the fueland oxidant source 202 may be converted to combustion products betweenthe input face 212 and the output face 214 of the perforated flameholder 102. According to an alternative interpretation, more than halfof the heat or thermal energy output by the combustion reaction 302 maybe output between the input face 212 and the output face 214 of theperforated flame holder 102. As used herein, the terms heat, heatenergy, and thermal energy shall be considered synonymous unless furtherdefinition is provided. As used above, heat energy and thermal energyrefer generally to the released chemical energy initially held byreactants during the combustion reaction 302. As used elsewhere herein,heat, heat energy and thermal energy correspond to a detectabletemperature rise undergone by real bodies characterized by heatcapacities. Under nominal operating conditions, the perforations 210 canbe configured to collectively hold at least 80% of the combustionreaction 302 between the input face 212 and the output face 214 of theperforated flame holder 102. In some experiments, the inventors produceda combustion reaction 302 that was apparently wholly contained in theperforations 210 between the input face 212 and the output face 214 ofthe perforated flame holder 102. According to an alternativeinterpretation, the perforated flame holder 102 can support combustionbetween the input face 212 and output face 214 when combustion is“time-averaged.” For example, during transients, such as before theperforated flame holder 102 is fully heated, or if too high a (cooling)load is placed on the system, the combustion may travel somewhatdownstream from the output face 214 of the perforated flame holder 102.Alternatively, if the cooling load is relatively low and/or the furnacetemperature reaches a high level, the combustion may travel somewhatupstream of the input face 212 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease ofdescription, it should be understood that in some instances, no visibleflame is present. Combustion occurs primarily within the perforations210, but the “glow” of combustion heat is dominated by a visible glow ofthe perforated flame holder 102 itself. In other instances, theinventors have noted transient “huffing” or “flashback” wherein avisible flame momentarily ignites in a region lying between the inputface 212 of the perforated flame holder 102 and the fuel nozzle 218,within the dilution region D_(D). Such transient huffing or flashback isgenerally short in duration such that, on a time-averaged basis, amajority of combustion occurs within the perforations 210 of theperforated flame holder 102, between the input face 212 and the outputface 214. In still other instances, the inventors have noted apparentcombustion occurring downstream from the output face 214 of theperforated flame holder 102, but still a majority of combustion occurredwithin the perforated flame holder 102 as evidenced by continued visibleglow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat fromthe combustion reaction 302 and output a portion of the received heat asthermal radiation 304 to heat-receiving structures (e.g., furnace wallsand/or radiant section working fluid tubes) in or adjacent to thecombustion volume 204. As used herein, terms such as radiation, thermalradiation, radiant heat, heat radiation, etc. are to be construed asbeing substantially synonymous, unless further definition is provided.Specifically, such terms refer to blackbody-type radiation ofelectromagnetic energy, primarily at infrared wavelengths, but also atvisible wavelengths owing to elevated temperature of the perforatedflame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputsanother portion of the received heat to the fuel and oxidant mixture 206received at the input face 212 of the perforated flame holder 102. Theperforated flame holder body 208 may receive heat from the combustionreaction 302 at least in heat receiving regions 306 of perforation walls308. Experimental evidence has suggested to the inventors that theposition of the heat receiving regions 306, or at least the positioncorresponding to a maximum rate of receipt of heat, can vary along thelength of the perforation walls 308. In some experiments, the locationof maximum receipt of heat was apparently between ⅓ and ½ of thedistance from the input face 212 to the output face 214 (i.e., somewhatnearer to the input face 212 than to the output face 214). The inventorscontemplate that the heat receiving regions 306 may lie nearer to theoutput face 214 of the perforated flame holder 102 under otherconditions. Most probably, there is no clearly defined edge of the heatreceiving regions 306 (or for that matter, the heat output regions 310,described below). For ease of understanding, the heat receiving regions306 and the heat output regions 310 will be described as particularregions 306, 310.

The perforated flame holder body 208 can be characterized by a heatcapacity. The perforated flame holder body 208 may hold thermal energyfrom the combustion reaction 302 in an amount corresponding to the heatcapacity multiplied by temperature rise, and transfer the thermal energyfrom the heat receiving regions 306 to heat output regions 310 of theperforation walls 308. Generally, the heat output regions 310 are nearerto the input face 212 than are the heat receiving regions 306. Accordingto one interpretation, the perforated flame holder body 208 can transferheat from the heat receiving regions 306 to the heat output regions 310via thermal radiation, depicted graphically as 304. According to anotherinterpretation, the perforated flame holder body 208 can transfer heatfrom the heat receiving regions 306 to the heat output regions 310 viaheat conduction along heat conduction paths 312. The inventorscontemplate that multiple heat transfer mechanisms including conduction,radiation, and possibly convection may be operative in transferring heatfrom the heat receiving regions 306 to the heat output regions 310. Inthis way, the perforated flame holder 102 may act as a heat source tomaintain the combustion reaction 302, even under conditions where acombustion reaction 302 would not be stable when supported from aconventional flame holder.

The inventors believe that the perforated flame holder 102 causes thecombustion reaction 302 to begin within thermal boundary layers 314formed adjacent to walls 308 of the perforations 210. Insofar ascombustion is generally understood to include a large number ofindividual reactions, and since a large portion of combustion energy isreleased within the perforated flame holder 102, it is apparent that atleast a majority of the individual reactions occur within the perforatedflame holder 102. As the relatively cool fuel and oxidant mixture 206approaches the input face 212, the flow is split into portions thatrespectively travel through individual perforations 210. The hotperforated flame holder body 208 transfers heat to the fluid, notablywithin thermal boundary layers 314 that progressively thicken as moreand more heat is transferred to the incoming fuel and oxidant mixture206. After reaching a combustion temperature (e.g., the auto-ignitiontemperature of the fuel), the reactants continue to flow while achemical ignition delay time elapses, over which time the combustionreaction 302 occurs. Accordingly, the combustion reaction 302 is shownas occurring within the thermal boundary layers 314. As flow progresses,the thermal boundary layers 314 merge at a merger point 316. Ideally,the merger point 316 lies between the input face 212 and output face 214that define the ends of the perforations 210. At some position along thelength of a perforation 210, the combustion reaction 302 outputs moreheat to the perforated flame holder body 208 than it receives from theperforated flame holder body 208. The heat is received at the heatreceiving region 306, is held by the perforated flame holder body 208,and is transported to the heat output region 310 nearer to the inputface 212, where the heat is transferred into the cool reactants (and anyincluded diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 210 is characterized by alength L defined as a reaction fluid propagation path length between theinput face 212 and the output face 214 of the perforated flame holder102. As used herein, the term reaction fluid refers to matter thattravels through a perforation 210. Near the input face 212, the reactionfluid includes the fuel and oxidant mixture 206 (optionally includingnitrogen, flue gas, and/or other “non-reactive” species). Within thecombustion reaction region, the reaction fluid may include plasmaassociated with the combustion reaction 302, molecules of reactants andtheir constituent parts, any non-reactive species, reactionintermediates (including transition states), and reaction products. Nearthe output face 214, the reaction fluid may include reaction productsand byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by atransverse dimension D between opposing perforation walls 308. Theinventors have found that stable combustion can be maintained in theperforated flame holder 102 if the length L of each perforation 210 isat least four times the transverse dimension D of the perforation. Inother embodiments, the length L can be greater than six times thetransverse dimension D. For example, experiments have been run where Lis at least eight, at least twelve, at least sixteen, and at leasttwenty-four times the transverse dimension D. Preferably, the length Lis sufficiently long for thermal boundary layers 314 to form adjacent tothe perforation walls 308 in a reaction fluid flowing through theperforations 210 to converge at merger points 316 within theperforations 210 between the input face 212 and the output face 214 ofthe perforated flame holder 102. In experiments, the inventors havefound L/D ratios between 12 and 48 to work well (i.e., produce low NOx,produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heatbetween adjacent perforations 210. The heat conveyed between adjacentperforations 210 can be selected to cause heat output from thecombustion reaction portion 302 in a first perforation 210 to supplyheat to stabilize a combustion reaction portion 302 in an adjacentperforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 canfurther include a fuel nozzle 218, configured to output fuel, and anoxidant source 220 configured to output a fluid including the oxidant.For example, the fuel nozzle 218 can be configured to output pure fuel.The oxidant source 220 can be configured to output combustion aircarrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holdersupport structure 222 configured to hold the perforated flame holder 102at a dilution distance D_(D) away from the fuel nozzle 218. The fuelnozzle 218 can be configured to emit a fuel jet selected to entrain theoxidant to form the fuel and oxidant mixture 206 as the fuel jet andoxidant travel along a path to the perforated flame holder 102 throughthe dilution distance D_(D) between the fuel nozzle 218 and theperforated flame holder 102. Additionally or alternatively (particularlywhen a blower is used to deliver oxidant contained in combustion air),the oxidant or combustion air source can be configured to entrain thefuel and the fuel and oxidant travel through the dilution distanceD_(D). In some embodiments, a flue gas recirculation path 224 can beprovided. Additionally or alternatively, the fuel nozzle 218 can beconfigured to emit a fuel jet selected to entrain the oxidant and toentrain flue gas as the fuel jet travels through the dilution distanceD_(D) between the fuel nozzle 218 and the input face 212 of theperforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one ormore fuel orifices 226 having an inside diameter dimension that isreferred to as “nozzle diameter.” The perforated flame holder supportstructure 222 can support the perforated flame holder 102 to receive thefuel and oxidant mixture 206 at the distance D_(D) away from the fuelnozzle 218 greater than 20 times the nozzle diameter. In anotherembodiment, the perforated flame holder 102 is disposed to receive thefuel and oxidant mixture 206 at the distance D_(D) away from the fuelnozzle 218 between 100 times and 1100 times the nozzle diameter.Preferably, the perforated flame holder support structure 222 isconfigured to hold the perforated flame holder 102 at a distance about200 times or more of the nozzle diameter away from the fuel nozzle 218.When the fuel and oxidant mixture 206 travels about 200 times the nozzlediameter or more, the mixture is sufficiently homogenized to cause thecombustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fueland oxidant source, according to an embodiment. A premix fuel andoxidant source can include a premix chamber (not shown), a fuel nozzleconfigured to output fuel into the premix chamber, and an oxidant (e.g.,combustion air) channel configured to output the oxidant into the premixchamber. A flame arrestor can be disposed between the premix fuel andoxidant source and the perforated flame holder 102 and be configured toprevent flame flashback into the premix fuel and oxidant source.

The oxidant source 220, whether configured for entrainment in thecombustion volume 204 or for premixing, can include a blower configuredto force the oxidant through the fuel and oxidant source 202.

The support structure 222 can be configured to support the perforatedflame holder 102 from a floor or wall (not shown) of the combustionvolume 204, for example. In another embodiment, the support structure222 supports the perforated flame holder 102 from the fuel and oxidantsource 202. Alternatively, the support structure 222 can suspend theperforated flame holder 102 from an overhead structure (such as a flue,in the case of an up-fired system). The support structure 222 cansupport the perforated flame holder 102 in various orientations anddirections.

The perforated flame holder 102 can include a single perforated flameholder body 208. In another embodiment, the perforated flame holder 102can include a plurality of adjacent perforated flame holder sectionsthat collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured tosupport the plurality of perforated flame holder sections. Theperforated flame holder support structure 222 can include a metalsuperalloy, a cementatious, and/or ceramic refractory material. In anembodiment, the plurality of adjacent perforated flame holder sectionscan be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W betweenopposite sides of the peripheral surface 216 at least twice a thicknessdimension T between the input face 212 and the output face 214. Inanother embodiment, the perforated flame holder 102 can have a widthdimension W between opposite sides of the peripheral surface 216 atleast three times, at least six times, or at least nine times thethickness dimension T between the input face 212 and the output face 214of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a widthdimension W less than a width of the combustion volume 204. This canallow the flue gas circulation path 224 from above to below theperforated flame holder 102 to lie between the peripheral surface 216 ofthe perforated flame holder 102 and the combustion volume wall (notshown).

Referring again to both FIGS. 2 and 3, the perforations 210 can be ofvarious shapes. In an embodiment, the perforations 210 can includeelongated squares, each having a transverse dimension D between opposingsides of the squares. In another embodiment, the perforations 210 caninclude elongated hexagons, each having a transverse dimension D betweenopposing sides of the hexagons. In yet another embodiment, theperforations 210 can include hollow cylinders, each having a transversedimension D corresponding to a diameter of the cylinder. In anotherembodiment, the perforations 210 can include truncated cones ortruncated pyramids (e.g., frustums), each having a transverse dimensionD radially symmetric relative to a length axis that extends from theinput face 212 to the output face 214. In some embodiments, theperforations 210 can each have a lateral dimension D equal to or greaterthan a quenching distance of the flame based on standard referenceconditions. Alternatively, the perforations 210 may have lateraldimension D less then than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably,each of the plurality of perforations 210 has a lateral dimension Dbetween 0.1 inch and 0.5 inch. For example the plurality of perforations210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as thetotal volume of all perforations 210 in a section of the perforatedflame holder 102 divided by a total volume of the perforated flameholder 102 including body 208 and perforations 210. The perforated flameholder 102 should have a void fraction between 0.10 and 0.90. In anembodiment, the perforated flame holder 102 can have a void fractionbetween 0.30 and 0.80. In another embodiment, the perforated flameholder 102 can have a void fraction of about 0.70. Using a void fractionof about 0.70 was found to be especially effective for producing verylow NOx.

The perforated flame holder 102 can be formed from a fiber reinforcedcast refractory material and/or a refractory material such as analuminum silicate material. For example, the perforated flame holder 102can be formed to include mullite or cordierite. Additionally oralternatively, the perforated flame holder body 208 can include a metalsuperalloy such as Inconel or Hastelloy. The perforated flame holderbody 208 can define a honeycomb. Honeycomb is an industrial term of artthat need not strictly refer to a hexagonal cross section and mostusually includes cells of square cross section. Honeycombs of othercross sectional areas are also known.

The inventors have found that the perforated flame holder 102 can beformed from VERSAGRID® ceramic honeycomb, available from AppliedCeramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to theinput and output faces 212, 214. In another embodiment, the perforations210 can be parallel to one another and formed at an angle relative tothe input and output faces 212, 214. In another embodiment, theperforations 210 can be non-parallel to one another. In anotherembodiment, the perforations 210 can be non-parallel to one another andnon-intersecting. In another embodiment, the perforations 210 can beintersecting. The body 308 can be one piece or can be formed from aplurality of sections.

In another embodiment, which is not necessarily preferred, theperforated flame holder 102 may be formed from reticulated ceramicmaterial. The term “reticulated” refers to a netlike structure.Reticulated ceramic material is often made by dissolving a slurry into asponge of specified porosity, allowing the slurry to harden, and burningaway the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, theperforated flame holder 102 may be formed from a ceramic material thathas been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include aplurality of tubes or pipes bundled together. The plurality ofperforations 210 can include hollow cylinders and can optionally alsoinclude interstitial spaces between the bundled tubes. In an embodiment,the plurality of tubes can include ceramic tubes. Refractory cement canbe included between the tubes and configured to adhere the tubestogether. In another embodiment, the plurality of tubes can includemetal (e.g., superalloy) tubes. The plurality of tubes can be heldtogether by a metal tension member circumferential to the plurality oftubes and arranged to hold the plurality of tubes together. The metaltension member can include stainless steel, a superalloy metal wire,and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stackedperforated sheets of material, each sheet having openings that connectwith openings of subjacent and superjacent sheets. The perforated sheetscan include perforated metal sheets, ceramic sheets and/or expandedsheets. In another embodiment, the perforated flame holder body 208 caninclude discontinuous packing bodies such that the perforations 210 areformed in the interstitial spaces between the discontinuous packingbodies. In one example, the discontinuous packing bodies includestructured packing shapes. In another example, the discontinuous packingbodies include random packing shapes. For example, the discontinuouspacking bodies can include ceramic Raschig ring, ceramic Berl saddles,ceramic Intalox saddles, and/or metal rings or other shapes (e.g. SuperRaschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systemsincluding the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as aheat source to maintain a combustion reaction even under conditionswhere a combustion reaction would not be stable when supported by aconventional flame holder. This capability can be leveraged to supportcombustion using a leaner fuel-to-oxidant mixture than is typicallyfeasible. Thus, according to an embodiment, at the point where the fuelstream 206 contacts the input face 212 of the perforated flame holder102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a(conventional) lower combustion limit of the fuel component of the fuelstream 206—lower combustion limit defines the lowest concentration offuel at which a fuel and oxidant mixture 206 will burn when exposed to amomentary ignition source under normal atmospheric pressure and anambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforatedflame holder 102 described herein were found to provide substantiallycomplete combustion of CO (single digit ppm down to undetectable,depending on experimental conditions), while supporting low NOx.According to one interpretation, such a performance can be achieved dueto a sufficient mixing used to lower peak flame temperatures (amongother strategies). Flame temperatures tend to peak under slightly richconditions, which can be evident in any diffusion flame that isinsufficiently mixed. By sufficiently mixing, a homogenous and slightlylean mixture can be achieved prior to combustion. This combination canresult in reduced flame temperatures, and thus reduced NOx formation. Inone embodiment, “slightly lean” may refer to 3% O₂, i.e. an equivalenceratio of ˜0.87. Use of even leaner mixtures is possible, but may resultin elevated levels of O₂. Moreover, the inventors believe perforationwalls 308 may act as a heat sink for the combustion fluid. This effectmay alternatively or additionally reduce combustion temperatures andlower NOx.

According to another interpretation, production of NOx can be reduced ifthe combustion reaction 302 occurs over a very short duration of time.Rapid combustion causes the reactants (including oxygen and entrainednitrogen) to be exposed to NOx-formation temperature for a time tooshort for NOx formation kinetics to cause significant production of NOx.The time required for the reactants to pass through the perforated flameholder 102 is very short compared to a conventional flame. The low NOxproduction associated with perforated flame holder combustion may thusbe related to the short duration of time required for the reactants (andentrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burnersystem including the perforated flame holder shown and described herein.To operate a burner system including a perforated flame holder, theperforated flame holder is first heated to a temperature sufficient tomaintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step402, wherein the perforated flame holder is preheated to a start-uptemperature, T_(S). After the perforated flame holder is raised to thestart-up temperature, the method proceeds to step 404, wherein the fueland oxidant are provided to the perforated flame holder and combustionis held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406,wherein start-up energy is provided at the perforated flame holder.Simultaneously or following providing start-up energy, a decision step408 determines whether the temperature T of the perforated flame holderis at or above the start-up temperature, T_(S). As long as thetemperature of the perforated flame holder is below its start-uptemperature, the method loops between steps 406 and 408 within thepreheat step 402. In step 408, if the temperature T of at least apredetermined portion of the perforated flame holder is greater than orequal to the start-up temperature, the method 400 proceeds to overallstep 404, wherein fuel and oxidant is supplied to and combustion is heldby the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least someof which may occur simultaneously.

Proceeding from step 408, a fuel and oxidant mixture is provided to theperforated flame holder, as shown in step 410. The fuel and oxidant maybe provided by a fuel and oxidant source that includes a separate fuelnozzle and oxidant (e.g., combustion air) source, for example. In thisapproach, the fuel and oxidant are output in one or more directionsselected to cause the fuel and oxidant mixture to be received by theinput face of the perforated flame holder. The fuel may entrain thecombustion air (or alternatively, the combustion air may dilute thefuel) to provide a fuel and oxidant mixture at the input face of theperforated flame holder at a fuel dilution selected for a stablecombustion reaction that can be held within the perforations of theperforated flame holder.

Proceeding to step 412, the combustion reaction is held by theperforated flame holder.

In step 414, heat may be output from the perforated flame holder. Theheat output from the perforated flame holder may be used to power anindustrial process, heat a working fluid, generate electricity, orprovide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Varioussensing approaches have been used and are contemplated by the inventors.Generally, combustion held by the perforated flame holder is very stableand no unusual sensing requirement is placed on the system. Combustionsensing may be performed using an infrared sensor, a video sensor, anultraviolet sensor, a charged species sensor, thermocouple, thermopile,flame rod, and/or other combustion sensing apparatuses. In an additionalor alternative variant of step 416, a pilot flame or other ignitionsource may be provided to cause ignition of the fuel and oxidant mixturein the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to bestable, the method 400 may exit to step 424, wherein an error procedureis executed. For example, the error procedure may include turning offfuel flow, re-executing the preheating step 402, outputting an alarmsignal, igniting a stand-by combustion system, or other steps. If, instep 418, combustion in the perforated flame holder is determined to bestable, the method 400 proceeds to decision step 420, wherein it isdetermined if combustion parameters should be changed. If no combustionparameters are to be changed, the method loops (within step 404) back tostep 410, and the combustion process continues. If a change incombustion parameters is indicated, the method 400 proceeds to step 422,wherein the combustion parameter change is executed. After changing thecombustion parameter(s), the method loops (within step 404) back to step410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if achange in heat demand is encountered. For example, if less heat isrequired (e.g., due to decreased electricity demand, decreased motivepower requirement, or lower industrial process throughput), the fuel andoxidant flow rate may be decreased in step 422. Conversely, if heatdemand is increased, then fuel and oxidant flow may be increased.Additionally or alternatively, if the combustion system is in a start-upmode, then fuel and oxidant flow may be gradually increased to theperforated flame holder over one or more iterations of the loop withinstep 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228operatively coupled to the perforated flame holder 102. As described inconjunction with FIGS. 3 and 4, the perforated flame holder 102 operatesby outputting heat to the incoming fuel and oxidant mixture 206. Aftercombustion is established, this heat is provided by the combustionreaction 302; but before combustion is established, the heat is providedby the heater 228.

Various heating apparatuses have been used and are contemplated by theinventors. In some embodiments, the heater 228 can include a flameholder configured to support a flame disposed to heat the perforatedflame holder 102. The fuel and oxidant source 202 can include a fuelnozzle 218 configured to emit a fuel stream 206 and an oxidant source220 configured to output oxidant (e.g., combustion air) adjacent to thefuel stream 206. The fuel nozzle 218 and oxidant source 220 can beconfigured to output the fuel stream 206 to be progressively diluted bythe oxidant (e.g., combustion air). The perforated flame holder 102 canbe disposed to receive a diluted fuel and oxidant mixture 206 thatsupports a combustion reaction 302 that is stabilized by the perforatedflame holder 102 when the perforated flame holder 102 is at an operatingtemperature. A start-up flame holder, in contrast, can be configured tosupport a start-up flame at a location corresponding to a relativelyunmixed fuel and oxidant mixture that is stable without stabilizationprovided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operativelycoupled to the heater 228 and to a data interface 232. For example, thecontroller 230 can be configured to control a start-up flame holderactuator configured to cause the start-up flame holder to hold thestart-up flame when the perforated flame holder 102 needs to bepre-heated and to not hold the start-up flame when the perforated flameholder 102 is at an operating temperature (e.g., when T≧T_(S)).

Various approaches for actuating a start-up flame are contemplated. Inone embodiment, the start-up flame holder includes amechanically-actuated bluff body configured to be actuated to interceptthe fuel and oxidant mixture 206 to cause heat-recycling and/orstabilizing vortices and thereby hold a start-up flame; or to beactuated to not intercept the fuel and oxidant mixture 206 to cause thefuel and oxidant mixture 206 to proceed to the perforated flame holder102. In another embodiment, a fuel control valve, blower, and/or dampermay be used to select a fuel and oxidant mixture flow rate that issufficiently low for a start-up flame to be jet-stabilized; and uponreaching a perforated flame holder 102 operating temperature, the flowrate may be increased to “blow out” the start-up flame. In anotherembodiment, the heater 228 may include an electrical power supplyoperatively coupled to the controller 230 and configured to apply anelectrical charge or voltage to the fuel and oxidant mixture 206. Anelectrically conductive start-up flame holder may be selectively coupledto a voltage ground or other voltage selected to attract the electricalcharge in the fuel and oxidant mixture 206. The attraction of theelectrical charge was found by the inventors to cause a start-up flameto be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electricalresistance heater configured to output heat to the perforated flameholder 102 and/or to the fuel and oxidant mixture 206. The electricalresistance heater can be configured to heat up the perforated flameholder 102 to an operating temperature. The heater 228 can furtherinclude a power supply and a switch operable, under control of thecontroller 230, to selectively couple the power supply to the electricalresistance heater.

An electrical resistance heater 228 can be formed in various ways. Forexample, the electrical resistance heater 228 can be formed fromKANTHAL® wire (available from Sandvik Materials Technology division ofSandvik AB of Hallstahammar, Sweden) threaded through at least a portionof the perforations 210 defined by the perforated flame holder body 208.Alternatively, the heater 228 can include an inductive heater, ahigh-energy beam heater (e.g. microwave or laser), a frictional heater,electro-resistive ceramic coatings, or other types of heatingtechnologies.

Other forms of start-up apparatuses are contemplated. For example, theheater 228 can include an electrical discharge igniter or hot surfaceigniter configured to output a pulsed ignition to the oxidant and fuel.Additionally or alternatively, a start-up apparatus can include a pilotflame apparatus disposed to ignite the fuel and oxidant mixture 206 thatwould otherwise enter the perforated flame holder 102. The electricaldischarge igniter, hot surface igniter, and/or pilot flame apparatus canbe operatively coupled to the controller 230, which can cause theelectrical discharge igniter or pilot flame apparatus to maintaincombustion of the fuel and oxidant mixture 206 in or upstream from theperforated flame holder 102 before the perforated flame holder 102 isheated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operativelycoupled to the control circuit 230. The sensor 234 can include a heatsensor configured to detect infrared radiation or a temperature of theperforated flame holder 102. The control circuit 230 can be configuredto control the heating apparatus 228 responsive to input from the sensor234. Optionally, a fuel control valve 236 can be operatively coupled tothe controller 230 and configured to control a flow of fuel to the fueland oxidant source 202. Additionally or alternatively, an oxidant bloweror damper 238 can be operatively coupled to the controller 230 andconfigured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operativelycoupled to the control circuit 230, the combustion sensor beingconfigured to detect a temperature, video image, and/or spectralcharacteristic of a combustion reaction held by the perforated flameholder 102. The fuel control valve 236 can be configured to control aflow of fuel from a fuel source to the fuel and oxidant source 202. Thecontroller 230 can be configured to control the fuel control valve 236responsive to input from the combustion sensor 234. The controller 230can be configured to control the fuel control valve 236 and/or oxidantblower or damper to control a preheat flame type of heater 228 to heatthe perforated flame holder 102 to an operating temperature. Thecontroller 230 can similarly control the fuel control valve 236 and/orthe oxidant blower or damper to change the fuel and oxidant mixture 206flow responsive to a heat demand change received as data via the datainterface 232.

FIG. 5 is a schematic diagram of a fuel and oxidant source 202 inconjunction with an electrically powered heater 502 combined with aflame holder body 208, according to an embodiment. The body 208 may bemade of a refractory material, such as for example ceramic, fire brick,etc., and may be part of an industrial furnace or heater. The body 208may, according to an embodiment, exclude metals such as stainless steel,except possibly as attachments or auxiliary parts (e.g., swirlers,vanes, supports, bolts, etc.) that are not part of the body 208 proper,which may be less than 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, or 0.01 of thebody structure or the body proper by weight; or in other words, the bodymay substantially comprise a non-metal or consist substantially ofnon-metallic material. The fuel and oxidant source 202 comprises a fueljet nozzle 218 configured to emit a jet of fuel and an oxidant 206 as acombined stream, for example, by the jet of fuel entraining air (orother oxidant). The fuel and oxidant source 202 may optionally beconfigured so as not to comprise or constitute a fuel/air mixer, i.e., adevice or system that premixes fuel and air for delivery to the regionof a combustion reaction 302.

The flame holder body 208 may be disposed adjacent to the fuel andoxidant stream 206 at a predetermined position at which it may beconfigured to initiate and maintain vortices of turbulence T in the fueland oxidant stream 206. Optionally, to induce more local turbulenceand/or vortices, and thereby assist in flame holding, the body 208 mayincludes a vortex generator 502, that may be shaped as a ramp orprojection as illustrated, or may be shaped in some other way that willproduce additional turbulence, or vortices, or otherwise assist withflame holding; or, the body 208 may act as a bluff body that inducesadditional turbulence without being shaped specifically for that purpose(e.g., without a specific vortex generator), but rather does so by beingplaced in a predetermined position in the flow of mixture and/or in thecombustion reaction or flame 302. The body may act either by disruptingstreamlines that would otherwise be present, or by the Coanda effect, orby its motion, or by some other mechanism.

A body that generates vortices and/or turbulence thereby promotes heattransfer between the body and fluid with which it interacts. The body208 may also increase such heat transfer by having a larger surfacearea. For example, if the vortex generator 502 were perforated withthrough-holes, then it might create more turbulence, but its heattransfer rate would be increased even if the amount of turbulence didnot increase, due to the larger surface area. Heat transfer between abody and fluid is a concern of the applicants.

While not shown in FIG. 5, it will be understood that the illustratedportion of flame-holder body 208 may be merely representative of adifferently shaped or longer body, and in particular might schematicallyrepresent a curved flame-holder body that curves circumferentiallyaround the combustion reaction 302, and or an axis of the nozzle 218.

The body 208 receives heat from an adjacent diffusion-limited combustionreaction or flame 302 supported by the fuel and oxidant stream 206. Theheat of the flame raises the body 208 to an elevated temperature, afterthe flame is well established and the body 208 has time to heat up.However, in the early stages of combustion, especially, the body 208 maynot have reached its steady state or equilibrium temperature, and as aresult the dynamics of the flame or reaction 302 are affected. To morequickly bring the body 208 to an operating temperature, and/or tomaintain that temperature during steady-state operation (or, to maintaina more-elevated temperature), an electrically powered heater 502 isarranged and deployed to conduct at least a majority of heatelectrically generated in the heater 502 into the flame holder body and,at most, a minority of the generated heat to the fuel and oxidant stream206.

The oxidant source 220 may be illustrated as a mere pipe conducting theoxidant. The oxidant may be air, although any oxidizer, such as gaseousoxygen or liquid oxidants may be used. The oxidant source 220 mayinclude a natural-draft or forced-draft arrangement, such as a blower506 configured to provide a flow of combustion air. Conventional devicesmay be used for the fuel and oxidant source 202.

In an embodiment, the electrically powered heater 502 constitutes anelectrical resistance heating element, and may be made from conventionalmaterials suitable for low or high-temperature devices, such as tungstenfor example; it may have insulation, like the burner of anelectric-stove element, or it may be bare or otherwise covered. In FIG.5, the two ends of the heater 502 are coupled, by aschematically-illustrated wire 508, to a voltage supply, power supply,or current source 510 (shown schematically by a battery symbol).

A control system 512 may include a controller 514 (which may be, forexample, a processor, relay arrangement, or other device), a temperaturesensor 234 that is coupled to the controller 514, and a device 516 forvarying the electric current through the heater element 502; in FIG. 5,this device is shown schematically as a variable resistance, but anyother current-varying device, such as a transistor, can be used. Thecontrol system may use input signals from the temperature sensor 234 forfeedback to control the current-control device 516; thus, in oneembodiment the control system 512 comprises a thermostat. Thetemperature sensor 234 may detect the temperature directly (as byinfrared detector, thermocouple, thermopile, or the like) or indirectly(as by combustion products).

A schematically-illustrated known electrically powered hot-surfaceigniter 518 is shown in FIG. 5, with a hot end thereof disposed in theflow of fuel and oxidant. The hot-surface igniter is not a flame holderand is used only for ignition of the fuel in the flow of entrained air(e.g., starting a flame). Because it only needs to heat a local regionto above the flammability point, a hot-surface igniter is often smalland does not affect the flow or create substantial vortices.

The flame holder body 208 may be disposed in relation to the heater 502in such a way that a majority of the heat generated by electric currentin the heater 502 flows into the flame holder body, rather than flowingdirectly into the environment of the flame holder body and theelectrically powered heater 502, and therefore bypassing the flameholder body. When a majority of the electric-current heat flows into thebody 208, then the body 208 will more quickly reach its desiredoperating temperature, and a steady-state dynamic of the combustionreaction 302 will more quickly be reached. If heat is not being removedfrom the combustion reaction 302 into the body 208, as it is when thebody 208 is cold, then the flame dynamic is not disturbed and, e.g.,flame “lifting” or even extinguishing of the flame, is made less likely.

Heat, as is well known in physics, moves from a place of highertemperature to a place of lower temperature via three primarymechanisms: conduction, convection, and radiation. In conduction, heatis transferred via molecular impacts; in convection, heat is transferredvia bulk motion of hot or cold fluid; and in radiation, heat istransferred via photons (electromagnetic radiation).

In all three types, the rate of heat transfer is substantially dependenton surface area. In radiation, for example, a hot object such as aglowing ember in a fire emits a certain amount of radiation, carrying acorresponding certain amount of heat energy away from the ember; and twosuch embers, having twice the surface area, will emit two times theamount of heat. Similarly, two similar cold objects in the path of theember's heat radiation will absorb twice the heat that one would. Inconduction, a similar correspondence exists, because heat transferthrough a slab of material is directly proportional to the area of thatslab (the other factors are the thickness, the heat conductivity of theslab material, and the temperature difference between the two sides).Convection is more complicated, because fluid flow is itself morecomplicated—the flows are usually deterministically chaotic, and heattransfer from a moving fluid to a surface depends on local flowconditions near the surface. However, the same general dependence onarea exists for convection also, in part because the heat path ofconvection usually starts and stops on respective solid surfaces thatbound the convective fluid.

FIG. 5 typifies that an electrically powered heater 502 may be disposedsubstantially within a flame holder body 208, which may be one way ofcausing heat to flow from the hotter body (the heated wire) to thecolder body (the flame holder), rather than to flow somewhere else(i.e., the environment, which may be the fuel and oxidant stream 206,ambient air, nearby or attached objects, etc.). If a majority of thearea of the heater is in contact with, or close to, the flame holderbody 208, this will tend to cause a majority of the heat generated inthe heater to flow directly into the flame holder body 208, by any orall of the three heat-transfer mechanisms.

However, this is not a necessary condition. In the case where anelectrically powered heater wire or elongated element is in contact witha planar part of the surface of the flame holder body (not shown in FIG.5), the body may not substantially surround the heater, but it may stillabsorb most of the heat generated in the heater because conductionthrough the contact region will pass more energy than radiation andconvection will pass to the environment.

One of the inventors' objects is to pre-heat a flame holder body, whichmay for example be done to quickly normalize a combustion process. In anongoing combustion process, the flame holder body has achieved a certainoverall or average temperature by absorbing heat generated in a flame,and this temperature may be a steady-state temperature if the fuel andoxidant flows are steady-state. However, when the flame holder body iscold, for example when combustion is started, the flame conditions aredifferent from these steady-state conditions; and then, adjustment ofthe flame conditions (for example by adjusting the fuel or oxidant flow)cannot be performed directly for steady-state conditions. Thesteady-state conditions corresponding to mixture flow settings will notresult from the adjustment, due to the cold flame holder; the operatormust guess. Thus, operation of a boiler, furnace, jet engine, or otherfuel-burning device, may be simplified by electrically heating a flameholder.

FIG. 6 illustrates an embodiment in which the flame-holding body may beassociated with a burner tile 602 that has an internal ridge or ledge604. The burner tile 602 is shown in cross section, as if sawn down itsaxis. Curved arrows around the ridge or ledge 604 indicate toroidalvortices which may comprise a substantially toroidal pattern of vortices(i.e., there is an overall toroidal swirl pattern regardless ofnon-toroidal turbulence superimposed on the more-prevalent toroidalswirl pattern). The ridge or ledge 604 comprises a ramp shape, which isone shape for a projection into the moving portion of a flame; thevortex generator 502 is another projection shape.

A burner tile can be described as a flame holder, since it affects theflow of fuel/oxidant mixture and because a flame may be attached to arim of the burner tile, except during the undesirable condition of flame“lift-off” (which a flame holder can prevent). A burner tile may alsoinclude specific flame-holding structures such as the illustratedinternal ledge 604 that acts as a flame holder, and may also include aseparate flame stabilizer or flame holder disposed within, that may beconsidered as a part of the burner tile (see, e.g., The John ZinkHamworthy Combustion Handbook, 2^(nd) ed., Vol. 2, FIGS. 6-10 and 6-11).

An electrically powered heater wire 502 is shown embedded inside theburner tile 602, in a position near to the ledge 604. In the illustratedpredetermined position, a majority of the heat generated in the heaterwire 502 will appear at the internal surface of the burner tile 602 andcause an elevated temperature at the ledge 604. The wire 502 may be castin place in the burner tile 602. If the burner tile 602 is assembledfrom segments, then the embedded section may be electrically connectedat their ends to form a complete circuit, or, a partial circuit may haveits ends connected to a current source. If the wire 502 forms a completecircuit, then current may be magnetically induced in it. The figureshows a multi-turn induction coil 606 on the outside of the burner tile602. If the material of the burner tile may be non-conductive (e.g.,non-metallic), then magnetic fields generated by the induction coil 606will induce current in the wire 502 when an AC voltage is impressedacross it. As in any transformer, more current will be induced in thecoil with fewer turns (wire 502 in FIG. 6), and a higher current causesmore electric heating. Thus, a multi-turn induction coil may be capableof inducing large heating effects in a single-turn coil such as wire502.

It may be noted that at least a portion of the body of the burner tile602 forms a conical surface, and the electrical heater wire 502 iswrapped around the conical interior surface, though separated from it(it may also be wrapped around an exterior conical surface, as are thewires of the coil 606 in FIG. 6). In alternative embodiments (notillustrated) the heater wire 502 may be deployed in contact with aconical surface of a refractory body, either on the outside of anexterior or interior conical surface, in a groove formed in an otherwiseconical surface, or against a ridge protruding from a conical surface.

According to various embodiments, the burner tile 602 may comprise adielectric body formed by casting refractory material a mold cavity (notshown), and the wire or wires 502 may be supported in the mold cavityduring the formation of the dielectric body. During such formation,refractory material can flow or pack around the wire 502, causing thewire 502 to be cast into the body of the burner tile 602 when therefractory material is hardened. The mold cavity can include at leastone via for wire supports and/or, when the burner tile 602 is assembledfrom segments, for portions of the wire 502 that extend to an outer orinner surface for electrical connection to the wire 502 of an adjacentburner tile segment (or to a power supply or current source even whenthere are no segments). In such a case, the wire 502 in each segmentmight be shaped as a partial arc of a circle, with radial extensions ateither end that protrude through the outer surface of the burner tile602 for electrical connection to the wire 502 of the adjacent segmentwhen the refractory material is hardened (not illustrated).

The heater wire 502 may be configured to provide tensile reinforcementof the dielectric body of the burner tile 602. According to variousembodiments, burner tile 602 segments can be formed in a mold cavitywith one or more inserts (not shown) configured to establish mechanicalfastener locations, as well as electrical connections. According tovarious other embodiments, the burner tile 602 or burner tile segmentscan be formed by sand casting the refractory material. The castrefractory material can include a cement-bonded material,phosphate-bonded materials, fiber reinforcement, and/or an aggregateparticle distribution.

In another embodiment, the refractory tile can include cast passages forallowing electrodes to be inserted, taken out, or interchanged asneeded. Cast passages can be formed during manufacturing of refractorytile according to the dimensions, shapes, and desired applications ofelectrodes within the structure. Cast passages can allow metal parts tobe inserted and taken out of the burner tile 602. In general, castablematerials can be used for a refractory body in order to minimize costwhen manufacturing complex shapes that can integrate one or moreelectrodes, resistance heating elements, or other parts.

Forming a refractory tile, with or without cast passages, can involveknown refractory manufacturing processes which can include mixing rawmaterials and forming into desired shapes and dimensions under wet ormoist conditions; followed by heating the refractory material to hightemperatures in a periodic or continuous tunnel kiln to form the ceramicbond that gives the refractory tile its refractory properties; andconcluding with a final processing stage that can involve milling,grinding, and sandblasting of the refractory tile.

In FIG. 6, the oxidant source or pipe 220 includes an air-flow control,shown as a damper plate 608 for example, while the fuel source orjet-forming pipe 218 includes a fuel-concentration adjustment (a devicefor varying the ratio of fuel and oxidant flowing into the combustionreaction 302, for example by varying the flow rate of fuel, of oxidant,or of both). In FIG. 6, the fuel-concentration adjustment is shown as anexemplary adjustable valve 610 (a variable-fuel-flow-rate pump/motor isan alternative; such a device can use a variable-speed motor and/or avariable-flow pump). The valve 610 can adjust the fuel concentrationeven without providing any airflow control such as the plate 608. Theseadjustment devices may, individually or in a combined manner, be coupledto respective actuators 612 and 614, which may be step motors, forexample, and the actuators 612 and 614 may in turn be coupled to acontrol system (they may also be manual, or both manual and automatic).Only one motor may be needed if the adjustment devices are combined, oris only one of them is provided. For example, they may be coupled bywires 616, 618 to the controller 514 of FIG. 5 (or some other automaticcontroller). The characteristics of the combustion reaction 302 aredetermined in part by the proportion and flow rates of the fuel andoxidant.

FIG. 7 is a cross-sectional diagram of a flame holder with an electricheater element inserted therethrough, according to an embodiment. FIG. 7relates to a flame holder body 208 that may be of arbitrary shape incross section and has an elongated central passage 702 through which theheater wire 502 passes. If the heater 502 passes along a centralpassage, then virtually all of the heat generated in the heater willpass into the body 208. FIG. 7 illustrates the fact that the flameholder body 208 is not limited to any specific shapes.

A particular example of such an arrangement might be in a gas fireplaceof the type in which a gas flame impinges on non-flammable (e.g.,ceramic) “logs.” These fireplaces require a warm-up period both toachieve mimicry of a wood fire, and also to begin to throw radiant heat,most of which comes from the heated “logs” once they are hot. If the“logs” include electric heaters according to the Applicants'construction, then the fireplace will more rapidly reach the desiredcondition of resembling a wood fire and giving off radiant heat. (The“logs” act as a flame holder because they give heat back to the fuel/airmixture, and thereby maintain complete combustion, and also becausetheir irregular shape causes turbulence. Such a “log” body 208, thoughnot commonly denoted as a “flame holder” in the art, is so termed hereinby the applicants and the phrase covers such a “log” in the followingclaims.)

In the case of the fireplace, and also in other cases such asmetallurgical and industrial applications, the flame holder mayeventually reach a temperature at which even high-temperature heatingelements deteriorate, oxidize, or even melt; the flame holder may beconstructed of ceramic, refractory material, or other non-metallicmaterials that can withstand temperatures higher than those tolerable tometals such as stainless steels. If so, then the heater element can besurrounded by a layer of insulation, anti-oxidant, or other covering 704as shown, but can also be exposed to a force flow of outside air. Forexample, if there is an air gap between the heater 502 and the passagewall, then air can be pumped through that passage, and keep thetemperature of the heater 502 below that of the flame holder body 208.The air can then be released into the environment, or recycled to an airpump. In a fireplace, a small air pump (not illustrated) can send airinto the “logs” by metal pipes to pass through the air gap passage andthence out into the fire.

FIG. 8 is a diagrammatic perspective view of a combustion system orburner system, according to an embodiment in which the flame holder body208 is embodied as a perforated flame holder, a flame holder includingperforations 210 that allow the passage of fuel and oxidant and/orcombustion reactants and/or combustion products from an input face 212of the perforated flame holder 102 to an output face 214 thereof, andwhich may support flames inside the perforations 210 of the perforatedflame holder 102. The perforations 210, and also the edges of theperforated flame holder 102, may induce turbulence in a flow of mixtureor combustion products.

In FIG. 8, the electrically powered heater 502 is exemplified by anelectrically resistive heating element in the form of a wire that isinterleaved in and out through some of the plurality of perforations 210(which may include through-bores or through-passages. The electricallypowered heater 502 may, for example, be formed from KANTHAL® wire(available from Sandvik Materials Technology division of Sandvik AB ofHallstahammar, Sweden) threaded through at least a portion of elongatedperforations 210 formed of the perforated flame holder 102.(Alternatively, the flame holder 102 can be heated by an inductiveheater, a high energy (e.g. microwave or laser) beam heater, africtional heater, or other types of heating technologies.)

The heating element 502 is, in an embodiment, operatively coupled to avoltage supply 510 via wires 508, and the voltage supply 510 may becoupled to a controller or control unit 514 by a bus, wire, or wirelessmeans. The control unit 514 may control the current flowing through theheating element 502 so as to vary the temperature of the perforatedflame holder 502, or portions thereof.

Although only a single heating element 502 is illustrated, it will beunderstood that plural heating elements 502 with associated plural pairsof wires 508 may be arranged from the voltage supply 510 (or fromrespective plural voltage supplies 510) all under the control of thecontroller 514, or, of plural controllers. In such an arrangement, thecontrol unit 514 may control the different heating units differentiallyor individually according to an algorithm, sequence, or timing schedule,and can also determine a distribution pattern of heat in the perforatedflame holder 102. The voltage supply 510 may be replaced by a currentsupply or some other electrical heat driver.

The nozzle 218 is shown located in an aperture 806 in a plate 808, whichschematically represents a furnace, or a furnace wall or wall portion.The nozzle 218 may, in an embodiment, be coaxial with an axis A of theperforated flame holder as shown. The nozzle 218 emits a fuel jet orstream 206 and is supplied from a fuel supply 802, a fluid line 804, anda fuel control valve 610, which may in an embodiment be under thecontrol of the control unit 514. (In an alternative embodiment, thevalve 610 includes a sensor that informs the control unit 514 of therate at which fuel is being delivered to the nozzle 218.) The controlunit 514 may further be coupled to a temperature sensor 234 that iscapable of detecting a temperature or temperature distribution of theupper second face 214 of the perforated flame holder 102 (for example,by infrared detection or imaging, although any other temperature orheat-measuring sensor may be used).

Because of these connections, the control unit 514 is capable, in anembodiment, of adjusting both the flow of fuel and the electricalheating of the perforated flame holder 102 in such as way as to maintaina predetermined temperature and/or temperature distribution of theperforated flame holder, or to make other adjustments related to thefuel rate and the temperature, such as controlling a rate of temperaturechange. Such adjustment can extend the turndown ratio, and/or compensatefor lower flame-holder temperature during cold starting.

During a startup procedure, the system control unit 514 may control thevoltage supply 510 to apply a voltage potential (or current) across theends of the heating element 502. The resistance value of the heatingelement 502 and the magnitude of the voltage potential may be selectedto generate sufficient heat to raise the temperature of the portion ofthe flame holder 102 in the vicinity of the heating element 502 tobeyond a startup threshold within a few seconds, after which the systemcontrol unit 514 controls valve 610 to open, while controlling thevoltage supply 510 to remove the voltage potential from the heatingelement 502. When the fuel stream 206 contacts the heated portion of theflame holder 502, auto-ignition occurs, and a stable flame isestablished in the flame holder 502.

The stream of fuel 206 ejected from the nozzle 218 toward the first face212 of the flame holder 102 disperses from the nozzle 218 in a conicalspray at an angle that is typically about 7.5 degrees from thelongitudinal axis A, resulting in a solid conical angle of about 15degrees. As the fuel stream 206 disperses, it entrains air, andeventually reaches a flammable proportion of fuel and air. By selectionof the nozzle orifice diameter and the pressure at which fuel isejected, the velocity at which the fuel stream 206 ejected from thenozzle 218 is preferably selected to be much higher than the flamepropagation speed of the particular type of fuel employed, so that, onthe one hand, the fuel stream is prevented from supporting a flame nearthe nozzle, and on the other hand, by the time the dispersing fuelstream 206 has slowed to near the flame propagation speed, the fuelstream has entrained enough air that the mixture is too lean forcombustion at the temperature of the fuel stream.

A nozzle-produced fuel jet such as 206 may have greater velocity nearthe axis A and may have fuel/oxidant ratios changing with the radialdistance from the axis A. Therefore, a wire heating element 502 may beplaced generally at a selected distance from the axis A where anignition temperature might be reached more quickly due to jetcharacteristics, or, where it might cause adjacent areas to more quicklyreach ignition temperature, or where some combination of flamecharacteristics are optimized. As mentioned above, plural heatingelements can be used, and these plural elements can be placed atgenerally different radial distances from the axis A.

After ignition and some time of combustion, the flame holder 102 is heldat a much higher temperature because of the ongoing combustion. Thehigher temperature of the flame holder 102 is sufficient to maintaincombustion of a lean fuel mixture. A stable flame can thus be maintainedby the flame holder 102. The flame is held primarily within theperforations 210, although the flame may extend a short distance beyondeither or both faces 212, 214 of the flame holder 102. The fuel stream206 is able to continually feed the combustion, and the flame holder 102is able to support a leaner flame than could be maintained in aconventional burner system.

The inventors have recognized that, although the flame holder 102 isable to support combustion with a very lean fuel mixture duringsteady-state operation, startup of combustion is problematic. The fuelmixture at any particular distance of the perforated flame holder 102from the nozzle 218 is flammable only at elevated temperatures, andbecause the flame holder 102 may be at ambient temperature at startup,conventional ignition methods or devices are not generally effective forstartup of combustion.

The applicants' apparatus and method are, however, also useful forsteady-start operation, following the start-up phase. This is becauseheating of the applicants' apparatus or via the applicants' methodscannot only more quickly achieve steady-state operation, but also canwiden the so-called “turndown ratio.” The turndown ratio may be definedas the ratio of maximum heat release to minimum heat release, withmaintenance of stable flames (see The John Zink Hamworthy CombustionHandbook, 2nd Ed., Volume 1, ¶10.7.1.3 and FIG. 17.15).

Loss of flame stability can result from too much or from too little fuelflow to a flame holder. Depending on the geometry of the flame holderand other factors such as the type of fuel, the flame will go out whenthere is not enough fuel to support the combustion (this may occur atthe lower end of the range of heat that defines the turndown ratio); andwhen there is too much flow, the flame will move up away from the flameholder (which may correspond to the upper end of the range of heat thatdefines the turndown ratio). In general, a flame will move “upstream” asthe fuel rate or flow rate decreases, and will move “downstream” as thefuel rate or flow rate increases.

As mentioned, too little fuel will hinder combustion. Aside from fuel,the other two main requirements of fire are oxidant and high-enoughtemperature. In a flame holder, the temperature of the fuel/air mixturemust be sufficient to reach combustion if the flame is to be stable. Ifthe flow is too great, then the temperature of the flame holder will belowered by the relatively cool un-burned mixture, and eventually theflame will leave the flame holder. Once that happens, flame stability isgreatly decreased. In general, a flame will move “upstream” as the fuelrate or flow rate decreases, and will move “downstream” as the fuel rateor flow rate increases.

The applicants electrically heat the flame holder at both ends of therange which determines the turndown ratio.

If, for example, the valve 610 informs the controller 514 that the fuelflow rate is too low to support a stable flame (or, when the controllershuts the valve to a certain point), then the controller may in responsesend electric current to the heating element 502, which will have theeffect of producing flame stability. As explained above, a leanermixture can be burned when the flame holder is heated.

If, on the other hand, the sensor 234 detects a drop in flame holdertemperature, perhaps due to too-rapid flow of unburned mixture, then thecontrol unit 514 might electrically heat the flame holder so as to bringthe combustion “upstream” and thereby prevent the flame from “lifting.”Or, the controller might electrically heat the flame holder whenever ithas instructed the valve 610 to open beyond a predetermined threshold.(The controller 514 may also detect fuel pressure and/or air flow anduse them as factors in determining when to initiate flame-holderheating.)

The discussion above is equally applicable to a perforated flame holderand a non-perforated flame holder, or to a flame holder that includes avortex generator 210 that is not a perforation.

FIG. 9 is a flow chart showing a method 900 for operating anelectrically heated burner, such as the electrically heated burner 100of FIG. 1, according to an embodiment. Beginning at step 902, start-upcommand data is received into a controller via a data interface. Thecontroller processes the instruction and, in step 904, enables a relayto output a drive voltage on at least one node of a power supply.Proceeding to step 906, the output drive voltage is coupled to anelectrically powered heater configured to cause a sensible temperaturerise in a component of an industrial or commercial burner, the componentbeing disposed so as to contact a fuel stream and/or a flame supportedby the fuel stream.

In step 908, the electrically powered heater causes a temperature risein at least a portion of a burner. Optionally, the method 900 caninclude step 910 to sense the temperature rise in the portion(s) of theburner. The method 900 can the proceed to optional step 912 to enable anignition source in the burner, and then to optional step 914 to enable afuel valve to output fuel at a fuel nozzle.

Proceeding to step 916, the electrically heated burner supportscombustion adjacent to the portion(s) of the burner.

FIG. 10 is a flow chart showing a method 1000 for operating anelectrically heated burner, such as the electrically heated burner 100of FIG. 1, according to another embodiment.

In step 916, combustion is supported adjacent to at least a portion ofan electrically heated industrial or commercial burner operativelycoupled to an electrically powered heater. In step 1002, current isoutput from a power supply to the electrically powered heater. Theoutput current causes the electrically powered heater to dissipateenergy, such as by electrical resistance or electrical inductance. Thedissipated energy enters the portion(s) of the electrically heatedburner.

Proceeding to step 1004, a temperature of the burner portion(s) issensed. Proceeding to step 1006, the current output to the electricallypowered heater is controlled to maintain the temperature. Additionallyor alternatively, the controller can operate a timer and causeapplication of electrical current to the electrically powered heateraccording to a duty cycle set point.

Proceeding to step 1008, burner temperature command data is received viaa data interface. Responsive to the command data, an electroniccontroller, in step 1010, changes a temperature set point for the burnerportion(s). In step 1012, the controller controls the current outputfrom the power supply to the electrically powered heater to change thetemperature of the burner portion to the new set point. Additionally oralternatively, the controller can change a heater duty cycle set pointand control the current output according to the new duty cycle setpoint. The new duty cycle set point can change the temperature of theburner portion(s).

Above, and in the following claims, “substantially within” indicatesthat, at room temperature or some other start-up temperature, most (amajority of) heat generated by the electrically powered heater due toelectric current flowing therethrough passes into the flame holder body,rather than passing directly into the environment (e.g., fuel/airmixture) while by-passing the flame holder body; or, conversely, that atmost a minority of electrically-generated heat passes directly to thefuel and oxidant stream.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments are contemplated. The various aspects andembodiments disclosed herein are for purposes of illustration and arenot intended to be limiting, with the true scope and spirit beingindicated by the following claims.

What is claimed is:
 1. A burner, comprising: a fuel and oxidant source configured to provide a fuel and oxidant stream that includes a range of fuel concentrations to support a combustion reaction; a body disposed adjacent to the fuel and oxidant stream and configured to exchange heat energy with the fuel and oxidant stream; and an electrically powered heater arranged to convey at least a portion of electrically generated heat to the body.
 2. The burner of claim 1, wherein the electrically powered heater is arranged to conduct a majority of electrically generated heat to the body.
 3. The burner of claim 1, wherein the body comprises a vortex generator.
 4. The burner of claim 1, wherein the body comprises a perforated flame holder body; and wherein being disposed adjacent to the fuel and oxidant stream comprises being disposed to receive the fuel and oxidant stream into a plurality of perforations defined by the perforated flame holder body.
 5. The burner of claim 1, wherein the electrically powered heater includes an electrical resistance heating element.
 6. The burner of claim 3, wherein the vortex generator is configured to generate vortices aligned circumferentially around the fuel and oxidant stream.
 7. The burner of claim 1, wherein the body comprises at least one non-flammable log.
 8. The burner of claim 1, further comprising: a thermostat operatively coupled to the electrically powered heater, the thermostat including a temperature sensor and an electronic controller operatively coupled to the temperature sensor; wherein the thermostat is configured to cause the electrically powered heater to maintain the body at a selected temperature.
 9. The burner of claim 8, wherein the electronic controller is configured to cause the electrically powered heater to raise the temperature of the body to the selected temperature before causing a fuel valve to open to provide the fuel and oxidant stream; and wherein the electronic controller is configured to cause the fuel valve to open to provide the fuel and oxidant stream after the temperature of the body reaches the selected temperature.
 10. The burner of claim 8, wherein the selected temperature is preselected to be equal to or greater than an autoignition temperature of the fuel.
 11. The burner of claim 8, wherein the thermostat is configured to increase a rate of heating by the electrically powered heater when a rate of fuel flow is reduced, and to decrease the rate of heating by the electrically powered heater when the rate of fuel flow is increased.
 12. The burner of claim 1, wherein the electrical heater includes at least a portion embedded in the body.
 13. The burner of claim 12, wherein the body defines one or more hollow passages formed within the body; and wherein the electrical heater is carried within the one or more hollow passages.
 14. The burner of claim 12, wherein at least a portion of the body forms a conical surface; and wherein the electrical heater is wrapped around the conical surface.
 15. The burner of claim 1, wherein the body defines one or more grooves formed in a surface of the body; and wherein the electrical heater is disposed in the one or more grooves.
 16. The burner of claim 1, further comprising: an electronic controller operatively coupled to the electrically powered heater, the electronic controller including a timer; and a fuel flow rate sensor operatively coupled to the electronic controller; wherein the electronic controller is configured to respond to the timer to cause the electrically powered heater to dissipate heat at a duty cycle inversely proportional to a rate of fuel flow sensed by the fuel flow rate sensor.
 17. The burner of claim 1, further comprising an igniter.
 18. The burner of claim 17, wherein the igniter is configured to be turned off after ignition.
 19. The burner of claim 17, wherein the electrically powered heater is configured to at least selectively remain powered during combustion.
 20. The burner of claim 17, wherein the igniter is separate and distinct from the electrically powered heater and the body.
 21. A method of increasing a turndown ratio of combustion, comprising: outputting fuel into a combustion volume; admitting air into the combustion volume; allowing the fuel to at least partially entrain the air in the combustion volume to form a fuel and air jet having a range of mixtures; supporting a flame holder adjacent to the fuel and air jet in a position predetermined to promote vortex formation in the mixture; igniting the fuel and air mixture to form a combustion reaction; and electrically heating the flame holder to maintain stable ignition of the fuel and air mixture within the vortices promoted by the flame holder.
 22. The method of combustion of claim 21, wherein maintaining ignition includes at least intermittently transferring electrically-generated heat from the flame holder to the vortices.
 23. The method of combustion of claim 21, wherein at least a majority of the electrically-generated heat flows to the turbulent portion of the mixture via a body of the flame holder.
 24. The method of combustion of claim 21, further comprising starting the combustion reaction with an igniter.
 25. The method of combustion of claim 21, wherein supporting a flame holder includes supporting a hollow cylindrical refractory body peripheral to a fuel nozzle and an air source; and wherein supporting a flame holder adjacent to the fuel and air jet in a position predetermined to promote vortex formation in the mixture includes supporting the hollow cylindrical refractory body to receive at least a portion of the fuel and air jet through the hollow portion of the cylinder and supporting a vortex-formation surface at an end of the flame holder perpendicular to a nominal direction of fuel and air flow.
 26. The method of combustion of claim 25, wherein the vortex-formation surface is configured to cause toroidal vortices to form such that hot combustion products are recycled from the combustion reaction to the fuel and air jet.
 27. The method of combustion of claim 25, wherein allowing the fuel to at least partially entrain the air in the combustion volume to form a fuel and air jet having a range of mixtures further comprises adjusting at least one of a fuel flow rate and an air flow rate.
 28. The method of claim 21, wherein electrically heating the flame holder includes passing an electric current through an electrically powered heater.
 29. The method of claim 21, wherein electrically heating the flame holder includes passing an electric current through the flame holder.
 30. A control system for controlling dynamics of a flame, comprising: a source of electric current; an electrically heated flame holder immersed in the flame and coupled to the source, wherein the source induces electric current flow therethrough; a temperature-responsive current controller coupled to the source and the electrically heated flame holder to adjust the electric current flow through the electrically heated flame holder toward a predetermined temperature of the electrically heated flame holder.
 31. The control system for controlling dynamics of a flame of claim 30, wherein the temperature-responsive current controller further comprises a temperature sensor.
 32. The control system for controlling dynamics of a flame of claim 30, wherein the temperature-responsive current controller further constitutes a thermostat.
 33. The control system for controlling dynamics of a flame of claim 30, wherein the temperature-responsive current controller further comprises a processor.
 34. The control system for controlling dynamics of a flame of claim 30, wherein the flame holder comprises at least one vortex-forming structure.
 35. The control system for controlling dynamics of a flame of claim 34, wherein the vortex-forming structure includes a projection into a moving portion of the flame.
 36. The control system for controlling dynamics of a flame of claim 35, wherein the projection further comprises a ramp.
 37. The control system for controlling dynamics of a flame of claim 30, wherein the control system further comprises an electrically heated surface igniter used only to start the flame. 